As a method for making a printing plate by forming concavities and convexities on a photosensitive resin layer layered on a support surface, a method where a relief forming layer formed of a photosensitive composition is exposed to UV-rays through an original image film, an image portion is selectively cured, and an uncured portion is removed with a developer, so-called “analogue plate making”, is well known.
A relief printing plate is an anastatic printing plate having a relief layer with concavities and convexities. The relief layer with concavities and convexities is obtained by patterning a relief forming layer containing a photosensitive composition containing, as a main component, an elastomer polymer such as a synthetic rubber, a resin such as a thermoplastic resin or a mixture of a resin and a plasticizer to form concavities and convexities. Among such relief printing plates, a relief printing plate having a soft relief layer is called a flexo printing plate in some cases.
When a relief printing plate is produced by analogue plate making, in general, an original image film formed with silver salt materials is necessary; accordingly, it takes a long time to produce an original image film, and the cost is high. Furthermore, a chemical process is necessary to develop an original image film, and disposal of a development waste liquid is also necessary. Accordingly, a method for more readily producing a printing plate such as a method that does not use an original image film or a method that does not require development is under study.
Recently, a method where plate making is performed by means of scanning exposure of a relief forming layer without using an original image film is under study.
For a method that does not require an original image film, a relief printing plate precursor where a laser-sensitive mask layer element capable of forming an image mask is disposed on a relief forming layer has been proposed (see, for example, U.S. Pat. No. 5,798,202 and Japanese Patent Application Laid-Open (JP-A) No. 2002-3665). According to these plate making methods, an image mask having the same function as that of an original image film is formed from the mask layer element by laser irradiation based on image data. Accordingly, the method is referred to as a “mask CTP method” and does not require an original image film. However, a subsequent plate making step is a step where UV-rays are irradiated through the image mask and an uncured portion is developed and removed. That is, there is still a room for improvement with respect to the point that a development process is necessary.
As a plate making method that does not require a development step, a so-called “direct engraving CTP method” where a relief forming layer is directly engraved with a laser to make a plate has been proposed many times. The direct engraving CTP method is literally a method of forming concavities and convexities, which will serve as a relief, by laser engraving. This method is advantageous in that the relief shape may be freely controlled, unlike the relief formation process using an original image film. For this reason, in the case of forming an image such as an outline character, the image region may be engraved deeper than other regions, and in the case of forming a microdot image, shouldered engraving in consideration of resistance to printing pressure may be carried out.
With respect to the plate materials that have been used in the direct engraving CTP method, various materials have been proposed such as those in which a hydrophobic elastomer (rubber) is used (see, for example, U.S. Pat. No. 5,798,202, JP-A No. 2002-3665, Japanese Patent No. 3438404, JP-A Nos. 2004-262135 and 2001-121833.), or a hydrophilic polyvinyl alcohol derivative is used (see, for example, JP-A No. 2006-2061.) as a binder that determines the characteristics of the plate materials.
When a hydrophobic polymer or elastomer (rubber) is used as a binder polymer in a relief forming layer, the resulting relief layer has good water resistance and, therefore, is highly resistant to aqueous inks during printing. However, when a relief forming layer containing a hydrophobic binder polymer is laser engraved, scraps generated by engraving form a sticky liquid, and the sticky liquid frequently makes a simple rinse operation with tap water difficult.
As a technique for improving rinse characteristics of the scraps after engraving, a technique where porous inorganic particles are contained in a relief forming layer and the liquid scraps are adsorbed by the porous inorganic particles to improve removability has been proposed (see, for example, JP-A No. 2004-174758.).